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Ohm's Law - It Really Works!Posted 2/10/1997By Kevin Caple
A local shop had a very unusual problem with an '89 tuned port Firebird. The car was towed in with a "cranks, but no-start condition." The tech working on it had 12 years GM experience and is very familiar with the F-body car line. He checked it out and found scan tool readings to be normal and no codes set. There was good spark at the plugs, good fuel pressure, and a fuel sample looked and smelled normal. He then found that he could start and run the car on carburetor spray, which meant the engine was mechanically OK and the exhaust was not restricted. Next, he installed a noid light on each bank and found he had no injector pulse, so he disconnected and individually ohm-tested the injectors. All were in normal range. He also had good power on one side of each injector harness connector. He then tapped the reference wire to the ECM with 5 volts at the 4-wire connector (module disconnected) to simulate the signal sent to the ECM by the module. Each time he tapped the wire, the fuel pump relay came on so he knew the ECM was seeing the reference pulse. He still had no pulse at his noid light. Before he replaced the ECM and updated the prom, he verified that he had good continuity on his wire harness back to the computer. Both wires checked OK, yet with the new ECM it was still a no-start, no-pulse condition. When he called me, he restated his tests. I asked him to re-check the Throttle Position Sensor (TPS) voltage, which he confirmed was at 0.6 volts. Next we went to the passkey theft deterrent system (after verifying his noid light did work), thinking that there could be a problem with the 5-volt square wave signal the ECM needs to allow injector operation. Luckily, he has a lab scope and found a good signal at the ECM on circuit 229 from the passkey decoder module. Next, I had him amp draw the injectors thinking they were testing OK with the ohm meter, but were drawing too much current. When electrically activated, they once again tested OK. Adding power to the injectors was of no help and re-testing the wires back to the ECM showed them to be good. We both knew we must have a wiring problem, but what were we overlooking? Next, he disconnected all the injectors and the ECM, then installed a fused jumper wire from battery positive to the wire that drives the right bank of injectors. Then, at the ECM, I had him make a test light out of an 1156 bulb, touching the wire with the test light at the ECM connector. Because the 1156 takes about 1 amp to operate, this would be a good way to verify the wire can actually flow current. This showed a nice bright light, so he then checked the left harness in the same manner and it also resulted in a brightly lit bulb. But wait, the tech had not moved the power to the other harness ... just the test light. Finally a light at the end of the tunnel ... or is it a train?! He hooked all the injectors back up and took an ohm reading at the computer connector. He had 0.2 ohms on each bank. Ohm's Law tells us that if each injector is 16 ohms and they're in a parallel circuit of four injectors, our total resistance should be no less than 4.0 ohms (16 _ 4 = 4). The lower the resistance, the higher the current flow, which the ECM monitors and shuts the injector driver down to protect itself. After repairing the wires (there were several melted together going through the firewall), the car started and ran fine. He installed the old ECM and prom, and said from now on he is going to start checking problem cars at the ECM so he can see what the ECM is indeed looking at.
5-volt Oxygen Sensor?Posted 2/10/1997By Greg Montero
In case you're not aware of it, the oxygen sensors used on 1987 to 1990 Jeep 4.0L engines, as well as the 3.0L Eagle Premier engines, are not voltage-generating sensors. These are titania-style sensors. As with zirconia-style oxygen sensors, by measuring the amount of oxygen present in the exhaust, the ECU can make the adjustments necessary to the pulse width of the injectors. The resistance element in the oxygen sensor is a titania semiconductor (see Fig. 2). A 5-volt reference is applied to a fixed resistor (inside the ECU) that is wired in series with the sensor. The ECU monitors the voltage drop across this resistor to determine if the mixture is rich or lean. Because of this characteristic, poor connections can easily cause false readings in the system. Changes in the voltage signal from the oxygen sensor serve as air/fuel indicators. The changes occur because this style sensor acts as a variable resistor. When the oxygen content is low (rich mixture), the voltage signal will be less than 2.5 volts. With a lean mixture (high oxygen content), the voltage signal will be more than 2.5 volts. This style sensor is a heated sensor. The heater is a 12-volt resistance-style element. Power is supplied from an oxygen sensor heater relay, which is controlled by the ECU. The relay is a normally closed type. When the ECU determines that the engine is up to operating temperature and there is enough exhaust flow to keep the sensor hot, the ECU supplies a ground to a relay coil, opening the heater circuit to the sensor. The resistance value is approximately 5-7 ohms. Remember, this is for the heating element only, not the titania element. The oxygen sensors on these engines are a very big input for fuel control. If the sensor gets slow, or worse yet, gets stuck, you will experience major driveability problems - anything from surging, poor idle to black smoke from the exhaust. As you can see, it is more than a trim device. Testing the sensor is like testing the regular sensor you've been used to, except everything you learned about oxygen sensors is backward! You can test the sensor by backprobing (or if you're lucky, you'll have a scan tool that will work on the system). The voltage should be fluctuating from 0-5 volts when the system is in closed loop. All other rules about oxygen sensors apply, such as response time, testing procedures and safeguards. Remember that if you artificially enrich the mixture, the voltage should drop low (less than 2.5 volts), and if you give the engine a big vacuum leak, the voltage should go high (more than 2.5 volts) - the high and low voltages are just the opposite of zirconia-style sensors.
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